1. Cross Reference to Related Application
This application discloses subject matter that is disclosed and claimed in copending application Ser. No. 07/804,872 filed on Dec. 6, 1991, entitled "Apparatus and Method for Measuring the Thickness of Thin Films" and assigned to the assignee hereof and which is hereby expressly incorporated by reference.
Also hereby expressly incorporated by reference are related copending applications, also assigned to assignee hereof, having U.S. Ser. Nos. 07/906,079 (filed Jun. 29, 1992) and 07/891,344 (filed May 29, 1992) respectively entitled "Apparatus and Method for Performing Thin Film Layer Thickness Metrology on a Thin Film Layer Having Shape Deformations and Local Slope Variations" and "Apparatus and Method for Performing Thin Film Layer Thickness Metrology By Deforming A Thin Film Layer Into A Reflective Condenser."
2. Field of the Invention
The present invention relates to an apparatus and method for measuring a thin film or layer thickness and, without limitation, to an electro-optical system which measures the thickness of an outer silicon layer of a silicon/silicon dioxide/silicon (Si/SiO.sub.2 /Si) structured semiconductor wafer.
3. Description of the Prior Art
The above cited copending application discloses and claims an invention that is especially practical for measuring the thickness of a silicon-on-insulator (SOI) semiconductor wafer which typically includes a Si/SiO.sub.2 /Si sandwich structure fabricated by growing a silicon dioxide film on one surface of each of two silicon wafers and bonding the two silicon dioxide film surfaces together at high temperature. It should be understood, however, that the earlier invention and the present invention can be used for measuring any number of layers providing only one layer thickness is unknown and the optical properties of all layers are accurately known. It should also be understood that other materials such as, for example, silicon nitride, may be used for the insulator material and that other materials may be used for the wafer material. In such an application, one of the two outer silicon surfaces of the sandwich structure is mechanically ground and polished to an average thickness of several microns. This mechanical process unfortunately results in large spatial variations in the thickness of this outer silicon layer over the surface of the wafer. To reduce these spatial variations, a thickness error map that indicates thickness non-uniformities of this outer silicon layer over the entire wafer surface, is required, for example, to initialize a further micropolishing process.
A sequence of measuring the spatial variations in the thickness of the outer silicon layer followed by thinning and smoothing this surface by micropolishing needed to be performed several times before the entire outer silicon layer achieves the desired thickness. In order to reduce costs and increase production, a measurement of at least 400 points on a wafer surface in 60 seconds is desirable.
Before the above cited invention was made, measuring instruments typically provided film thickness measurements at only a single point on a surface. These instruments use a focused lens or a fiber bundle to locally illuminate the film surface with a beam of monochromatic light, and a grating or prism spectrograph to measure the surface spectral reflectance at each point. In all cases, this surface spectral reflectance data must be numerically corrected due to variations in the angle of incidence caused by the illuminating beam f-number.
These commercial instruments may be extended to cover an entire wafer surface by moving either the measuring instrument or the wafer in a controlled manner. However, the time required for these instruments to determine the thin film layer thickness at a single point is on the order of several minutes and characterizing an entire film surface of at least 400 measurement points far exceeds the time desired for efficient wafer production.
The above cited application disclosed an electrooptical imaging system for efficiently determining a thin film layer thickness of, for example, a wafer over a full aperture. Non-uniformities in this layer thickness are obtained by measuring the reflectance characteristics for a full aperture of a wafer surface and comparing this measured reflectance data to reference reflectance data by using numerical iteration or by using a calibration wafer having known layer thicknesses.
To efficiently measure the reflectance characteristics of a wafer layer, according to the above cited application, a filtered white light source is used to produce a sequence of collimated monochromatic light beams at several different wavelengths. These collimated monochromatic beams are individually projected onto the entire surface of the wafer, and coherent interactions occur between this light as it is reflected from the physical boundaries in the wafer structure. As a result of these interactions an interference fringe pattern is formed on the surface of the wafer for each projected beam and, consequently, for each wavelength. A reflected image of each fringe pattern is projected onto a detector array of, for example, a charge coupled device (CCD) camera, where the full aperture of this image is then captured. The fringe pattern image is captured by digitizing pixels in the CCD camera detector array corresponding to the image present. A reflectance map of the entire wafer surface is generated from this captured fringe pattern image. Several reflectance maps are generated from each measured wafer to eliminate thickness ambiguities which may result from outer layers having phase thicknesses greater than 2.pi..
The reference reflectance data for a wafer, as mentioned above, may be obtained, according to the above cited application, by a theoretical calculation or through the use of a calibration wafer. The theoretical method consists of numerically computing reference reflectance characteristics based on assumed values for the intrinsic optical properties of the wafer materials. Alternatively, a calibration wafer, having a known thickness profile, may be constructed from the same batch of materials used to construct the wafer to be measured. By subjecting this calibration wafer to the measuring method of the present invention, reference reflectance data is obtained for the known wafer.
According to the teachings of the above cited application, the comparison between the measured reflectance data and the reference reflectance data can then be performed by a computer. Upon performing this comparison, the computer can provide a mapping of layer thickness or a mapping of layer thickness nonuniformities over a full aperture of the wafer.
The silicon-on-insulator (SOI) wafers that are measured, consist of two silicon wafers sandwiching a thin layer of silicon dioxide and typically suffer mechanical distortion caused by the manufacturing and polishing process. This results in surface deformations of 50 to 100 microns and local slope changes up to 1/4 a degree. According to the above cited application, measurement of the thickness of the outer film of silicon requires forming images of the wafer on a CCD camera at various monochromatic wavelengths in the visible region. The images are digitized (512.times.512 pixels) and this data is used to derive the spatial variations of the wafer reflectance caused by differing thicknesses of the silicon film.
According further to the above cited application, a library of reflectance values for different values of the outer film thickness is precalculated at all the different wavelengths and is used to find a match between the measured sampled spectral reflectance and the precalculated sampled spectrum by using a least squares fitting technique. This calculation requires the actual reflectances of the SOI wafer to be derived from the digitized data before the least squares fitting technique can be carried out. However, the intensities in the digitized images are not only proportional to the wafer reflectances but also to the spectral properties of the light source, the camera and the coatings in the optical system. Thus, an absolute measure of the SOI wafer reflectance can be obtained by recording an additional set of images of a bare silicon wafer, and since the reflectance of bare silicon is accurately known, then the wafer reflectance can be scaled from the data in the two sets of images.
The use of two sets of measurements at different times requires that the two wafers being accurately aligned in position and angle. The light source must also be stable in amplitude over the measurement time of about one minute.